Compound semiconductor monolithic microwave integrated circuits (MMICs) have been widely applied in mobile communications and sensor devices in recent years, and thereby the demand for high integration and high performance MMICs is growing. Conventionally, the electronic components in a MMIC, such as transistors, capacitors, resistors, inductors, and transmission lines, are disposed in a two-dimensional manner. To increase the device integration, 3D MMICs with passive components placing over compound semiconductor devices in a three dimensional manner are developed. In compound semiconductor MMICs, Au is commonly used for the passive components and interconnects to prevent cross contamination. However, the high cost of Au limits the thickness of the passive components. For transmission lines, as an example, a Au transmission line used in the 3D MMICs with the width narrower than that used in the conventional MMICs can lead to a higher resistance, leading to signal loss and excess noise. The circuit performance, such as the power gain and the noise figure in a power amplifier, is degraded due to the increased resistance of transmission lines. To improve the circuit performance, the thickness of Au layer must be increased, which in turn significantly increases the overall manufacturing cost. The circuit performance using Au metal layers is therefore restricted by the price of Au. The price of Cu is much lower comparing with Au, and the electrical and the thermal conductivity of Cu is also much better. Therefore, it is preferred to develop 3D MMIC structure in which Au is replaced by Cu.